A double-acting piston arrangement and/or double-acting displacement arrangement within a closed working system for a Stirling type engine has been found to be advantageous for use in a compact high-specific output engine; there is only one principal moving part per cycle. In a four cylinder Stirling-type engine equipped with double-acting pistons, each cylinder is divided by the piston to comprise a hot space and a cold space. The hot space of one cylinder is connected by a heater, regenerator and cooler assembly with the cold space of the next most adjacent cylinder. This type of arrangement delivers more work to the engine shaft than that which is used to provide compression of the working medium, provided the variations of the volume in the hot spaces are sufficiently advanced in phase with respect to the variations in the cold spaces. Most notably, in a double-acting piston arrangement, the piston transmits energy from the work medium to the crank shaft not only during the down stroke, but while on the up stroke; each piston is situated between two systems. This is not true of a single acting piston arrangement within the Stirling engine. The upward stroke of the double-acting piston coincides for a large part with the expansion of the system preceding the piston and with the compression of the system downstream of the piston; conversely the downstroke coincides for a large part with the expansion of the downstream system and the compression with the upstream system. With a four cylinder double-acting type engine, there should be a phase shift of 90.degree. in the motions of the pistons. Volume variations of the corresponding hot and cold spaces then likewise will differ 90.degree. in phase. Of course, combinations can also be made with more than four systems and with different phase relationships. Within certain limits, this has little effect upon the efficiency of the engine, since the curve representing the efficiency of the hot gas process is a function of the phase difference between the hot and cold spaces and is fairly constant near maximum. The method of communication between the hot and cold spaces must be such that volume variations of the hot space must occur before volume variations of the cold space for the same thermodynamic unit. The order of piston movements determines the direction of rotation of the engine. Depending upon the power output of the engine desired, the multiple number of cylinders can be arranged in a variety of patterns including in-line, v-shape, star-shape and square. With the latter, a swashplate mechanism is suitable as the output drive.
In spite of the obvious advantages of the double-acting piston arrangement of a Stirling engine, there arises a critical problem during stalling of the engine. Both sides of the same piston are utilized for purposes of serving two distinct and separate thermodynamic units. During cold start up or following an engine stall during operation, double-acting piston engines characteristically will have equal pressures in the upper and lower portion of the cylinder. However, the surface areas over which the equalized pressures act are different. This results from the fact that the piston rod is typically attached to one side of any given double-acting piston. The net areas exposed to the working gas will be unequal due to the subtraction of the area occupied by the piston rod. Under certain conditions, primarily during an engine stall at high mean system pressure, restarting of the engine can become impossible due to the unbalance of forces across the piston surfaces.